1. Field of the Invention
This invention relates to a light emitting device provided with a nitride semiconductor (for example, InxAlyGa1xe2x88x92xxe2x88x92yN, 0xe2x89xa6x, 0xe2x89xa6y, x+yxe2x89xa61) including light emitting devices such as LED (light emitting diode) and LD (laser diode).
2. Prior Art
Recently, the light emitting device including nitride semiconductor layers such as a blue LED or LD has attracted attention. Since the device made of nitride semiconductor has a high melting temperature and a relatively high heat resistance, it has a small temperature dependence and is expected to find application in not only light emitting devices but also various kinds of devices.
Also, the LED has excellent properties such as low power consumption and long lifetime and has an effect on the economy of power consumption and the decrease of maintenance frequency. Therefore, the LED holds promise as a light emitting source for the traffic signals and a high bright LED which can give sufficient visual identification outdoors is expected to be developed. The light in the yellow region is, often used in an indicator that attracts attention such as directional signals of the car and a bulletin board for traffic information other than the aforementioned traffic signal.
The LED made of AlGaInP has been already realized as the high bright LED which emits the light having a wavelength in the yellow region. However, the LED made of AlGaInP had large temperature dependence and particularly, the emission output thereof decreased extremely at an elevated temperature. Such a decrease of the emission output at an elevated temperature becomes a large problem, particularly in the indicator which is mounted outdoors. This is because generally, the temperature within the indicator is very high in summer when the solar radiation is very strong or in the regions such as tropic zones and the decrease of the emission output in the situations that the solar radiation is very strong leads to the decrease of the visual identification. Such a fact becomes an extremely large problem for the indicator which emits light in the yellow region and is often used as an indicator that attracts attention.
It is known that the active layer is made of mixed crystal containing indium (In) and the mixing ratio x of In is large to decrease the band gap energy, with the result that the wavelength of the emitted light is long, so as to emit the light having a wavelength in the yellow region corresponding to yellow of the traffic signals in the light emitting device including nitride semiconductor layers (for example, InxAlyGa1xe2x88x92xxe2x88x92yN, 0xe2x89xa6x, 0xe2x89xa6y, x+yxe2x89xa61) However, if the wavelength of the emitted light is made long by increasing the mixing ratio of indium in the active layer to emit the light having a wavelength in the yellow region using the light emitting device having nitride semiconductor layers, the problem of decrease the emission output with increase of the mixing ratio of indium comes up. The emission output decreases markedly from the point where the emitting wavelength xcexd is about 550 nm. Further, the active layer including indium (In) in the amount required to achieve an emitting wavelength of about 590 nm has an extremely bad surface state and then, the problem that the semiconductor layers having a good crystallinity cannot be formed on such a surface thereof comes up.
This invention has been accomplished to solve the above-mentioned problems. It is an object of the present invention to enhance the emission output of the light emitting device including an active layer made of nitride semiconductor containing In, particularly, of the light emitting device which emits a light having a wavelength longer (not less, than 550 nm) than that in the yellow region. It is another object of the present invention to enhance the crystallinity of the semiconductor layers which are formed on the nitride semiconductor layer which emits the light having a wavelength in the yellow region.
To achieve the above-mentioned object, according to the present invention, there is provided a light emitting device having an active layer between the n-type semiconductor layer and the p-type semiconductor layer, characterized in that the active layer comprises an well layer made of Inx1Ga1xe2x88x92x1N (x1 greater than 0) containing In and a first barrier layer made of Aly2Ga1xe2x88x92y2N (y2 greater than 0) containing Al formed on the well layer.
In the light emitting light device having such a structure according to the present invention, the emission output of the nitride semiconductor light emitting device, particularly of the light emitting device comprising a nitride semiconductor layer which emits a light having a wavelength longer than that in the yellow region can be enhanced. Also, the semiconductor layers formed on the well layer and the first barrier layer can be improved in crystallinity.
Also, in the light emitting device according to the present invention, the well layer is made of ternary mixed crystal, Inx1Ga1xe2x88x92x1N (0.6xe2x89xa6x1xe2x89xa61), resulting in that the well layer and the first barrier layer formed thereon can be improved in crystallinity and the semiconductor layers formed on the first barrier layer can be further improved in crystallinity.
In the light emitting device according to the present invention, the well layer may contain Al that is diffused from the adjacent layer.
In the light emitting device according to the present invention, the first barrier layer may contain In that is diffused from the adjacent layer.
In the light emitting device according to the present invention, the mixing proportion x1 of In in the well layer is controlled to be not less than 0.6, resulting in the light emission of a wavelength in the yellow region or longer.
Also, in the light emitting device according to the present invention, the mixing proportion x1 of In in the well layer is preferably adjusted to obtain the light emission having a wavelength of not less than 530 nm.
Further, in the light emitting device according to the present invention, the mixing proportion y of Al in the first barrier layer is preferably not less than 0.1, resulting in, particularly, the improvement of the emission output of the device which emits the light having a wavelength in the yellow region or longer.
And the mixing proportion y of Al in the first barrier layer is more preferably adjusted to be not less than 0.15, most preferably not less than 0.2.
Thus, the increase of the mixing proportion y2 of Al in the first barrier layer enables the threshold voltage to be decreased.
And in the light emitting device according to the present invention, the active layer preferably comprises a second barrier layer made of Inx3Aly3Ga1xe2x88x92x3xe2x88x92y3N, (0xe2x89xa6x3xe2x89xa60.3, 0xe2x89xa6y3xe2x89xa60.1, x3+y3xe2x89xa60.3) and thereby, the well layer formed thereon can be improved in crystallinity.
And in the light emitting device according to the present invention, the second barrier layer is more preferably made of ternary mixed crystal, Inx3Ga1xe2x88x92x3N (0xe2x89xa6x3xe2x89xa60.3) or binary mixed crystal, GaN which corresponds to x3=0 and thereby, the crystal defects due to the difference in the lattice constant between the barrier layer and the well layer can be decreased and the deterioration of the crystallinity of the second barrier layer itself due to high mixed crystallization of In can be prevented, resulting in the improvement of the well layer in cystallinity.
Since the light emitting device according to the present invention comprises the second barrier layer, the well layer and the first barrier layer, even when the mixing proportion of In is large, the active layer having a good crystallinity can be formed. Such an active layer may be more suitable for the active layer in the multi-quantum-well structure.
And in the light emitting device according to the present invention, it is preferable that the n-type semiconductor layer comprises an n-type cladding layer to confine the carrier within the active layer and the p-type semiconductor layer comprises a p-type cladding layer to confine the carrier within the active layer, the device comprising an n-side second cladding layer made of nitride semiconductor containing In between the active layer and the n-type cladding layer and a p-side second cladding layer made of nitride semiconductor containing In between the active layer and the p-type cladding layer.
In the light emitting device in such a configuration, the n-side second cladding layer and the p-side second cladding layer can prevent the deterioration of the crystallinity and the occurrence of the undesirable distortion in the active layer due to the difference in the lattice constant between the n-type cladding layer and the p-type cladding layer, resulting in the enhancement of the emission output.
As described above, according to the present invention, the emission output of the nitride semiconductor device, particularly the,nitride semiconductor light emitting device which emits the light having a wavelength longer than that in the yellow region (not less than 550 nm) can be enhanced. The cystallinity of the nitride semiconductor layers on the nitride semiconductor layer from which the light having a wavelength in the yellow region is emitted can be improved.
Particularly, in the case that the mixing proportion of Al, y2 in the first barrier layer is not less than 0.1, that is, y2xe2x89xa70.1, the above-mentioned effect is brought to the fore. Further, the effect is also produced on the decrease of the threshold voltage of the light emitting device on condition that y2xe2x89xa70.15, preferably y2xe2x89xa70.2.